In a nuclear magnetic resonance imaging (hereinafter referred to as "MRI") apparatus for obtaining a tomogram of a desired portion of a subject (human body) by utilizing a nuclear magnetic resonance (hereinafter referred to "NMR") phenomenon, this invention relates to a method of irradiating a gradient magnetic field employing gradient moment nulling effect for rephasing those phases which are dephased by the movement of nuclei spinning (hereinafter defined as "nuclear spins") in a pulse sequence of the irradiation of a gradient magnetic field for imaging at a predetermined rate, and for reducing artifacts resulting from the movement and flow, and to an MRI apparatus using this method. More in particular, the present invention relates to a gradient magnetic field irradiation method and a controller therefore capable of preventing free induction decay signals (FID signals) occurring due to incompleteness of 180.degree. excitation radio frequency (RF) pulses particularly when applied to a spin echo method, from being rephased and superposed with original echo signals during signal sampling and thus resulting in artifacts.
The MRI apparatus measures a density distribution, a relaxation time distribution, etc, of atomic nuclei at a desired inspection portion of a subject by utilizing the NMR phenomenon, and obtains and displays an image of an arbitrary section or slice of the subject from the measurement data.
FIGS. 1 and 2 of the accompanying drawings illustrate the timing of the irradiation of frequency encoding direction gradient magnetic fields when a gradient moment nulling method is applied in a frequency encoding direction so as to rephase the phases of the dephased spins moving at a predetermined rate in a pulse sequence of a spin echo method as one of the measuring sequences used in the MRI apparatus described above.
In the pulse sequence shown in FIG. 1, after a 90.degree. excitation RF pulse (hereinafter referred to as the "90.degree. pulse") is irradiated as shown in FIG. 1(a), a negative gradient magnetic field pulse 21 is irradiated as shown in FIG. 1(b) and immediately thereafter, a gradient magnetic field pulse 22 having a positive intensity and area (expressed by intensity x irradiation time) equal to those of the gradient magnetic field pulse 21 are irradiated. Next, a 180.degree. excitation RF pulse (hereinafter referred to as the "180.degree. pulse") is irradiated as shown in FIG. 1(a) and then a negative gradient magnetic field pulse 23 is irradiated as shown in FIG. 1(b). At this time, the intensity and area of this gradient magnetic field pulse 23 are equal to those of the gradient magnetic field pulse 21 described above. Immediately thereafter, a positive gradient magnetic field 24 which is to serve as a signal readout gradient magnetic field pulse is irradiated. The intensity of this gradient magnetic field pulse 24 is equal to that of the gradient magnetic field pulse 21 and its area is twice that of the gradient magnetic field pulse 21. The interval Ti.sub.1 from the irradiation timing of the 90.degree. pulse to the irradiation timing of the 180.degree. pulse is equal to the interval Ti.sub.2 from the irradiation timing of the 180.degree. pulse to the center of the positive gradient magnetic field pulse 24.
In the pulse sequence shown in FIG. 2, after the 90.degree. pulse and the 180.degree. pulse are irradiated as shown in FIG. 2(a), a positive gradient magnetic field pulse 25 is irradiated as shown in FIG. 2(b) and immediately thereafter, a negative gradient magnetic field pulse 26 is irradiated and furthermore, a positive gradient magnetic field pulse 27 to serve as a signal readout gradient magnetic field pulse is irradiated immediately after the irradiation of the pulse 26. At this time, the intensity of each of these gradient magnetic field pulses 26 and 27 is equal to that of the first gradient magnetic 25. The interval Ti.sub.1 from the irradiation timing of the 90.degree. pulse to the irradiation timing of the 180.degree. pulse is equal to the interval Ti.sub.3 from the irradiation timing of the 180.degree. pulse to the center of the positive gradient magnetic field pulse 27.
In this case, in order to keep the rephasing effect unaltered with the change of the echo time, it has been customary to irradiate the frequency encoding direction gradient magnetic field Gf shown in FIG. 1(b) or FIG. 2(b) to the irradiation of the 90.degree. pulse and the 180.degree. pulse shown in FIGS. 1(a) and 2(a), and to insert one-half of the extension of the echo time into the position 1 immediately before the negative gradient magnetic field pulse 21 or into the position 2, immediately after the positive gradient magnetic field pulse 22 in FIG. 1(b), and the remaining half of the extension time, into the position 3, immediately before the next negative gradient magnetic field pulse 23.
Here, the phase change .phi. of the nuclear spin moving at a predetermined rate is expressed generally by the following formula (1): ##EQU1## where G: intensity of gradient magnetic field,
.gamma.: gyromagnetic rotation ratio, PA1 Xo: initial position of nuclear spin, PA1 v: moving velocity of nuclear spin, PA1 t.sub.0, t.sub.1 : irradiation start and stop timing of gradient magnetic field.
As a result, the phase change of the nuclear spin, that forms the original echo signal 28 shown in both of FIGS. 1(c) and 2(c), returns to zero at the peak position of the echo signal 28 (this is referred to as "rephase") as represented by solid line 29 (representing the moving nuclear spin) and dash line 30 (representing the stationary nuclear spin) in (d) of both FIGS. 1 and 2, and the echo signal 28 free from phase distortion can be obtained.
The solid line 29 represents the case where the nuclear spin moves towards a higher magnetic field intensity, and is a secondary function due to the term vt in the formula (1). Since the stationary spin corresponds to the case of v=0 in the formula (1), the phase change is a primary function as represented by the dash line 30.
In the pulse sequence in the gradient magnetic field irradiation method described above, the relation of the gradient magnetic field pulse at the peak position of the echo signal 28 from the excitation of the nuclear spin by the 180.degree. pulse, i.e. the relation of the gradient magnetic field pulse (gradient magnetic field intensity x irradiation time) till the center point of the signal sampling time, is such that [the area of the gradient magnetic field 23] = [the rear of the hatched portion of the gradient magnetic field pulse 24] in FIG. 1(b), while [the area of the gradient magnetic field pulse 25 + the area of the hatched portion of the gradient magnetic field pulse 27] = [the rear of the gradient magnetic field pulse 26] in FIG. 2(b). Therefore, when a free induction decay (hereinafter referred to as "FID") signal 31 excited by the 180.degree. pulse occurs due to incompleteness of the 180.degree. pulse as shown in FIG. 1(e) or 2(e), the phase of the stationary nuclear spin of the FID signal 31 changes in accordance with the principle of the gradient echo method as shown in FIG. 1(f) or 2(f), becomes zero at the center of the sampling time, and becomes an echo signal 32 having its peak at the same position as the peak of the original echo signal 28 excited by the 90.degree. pulse. In this instance, the echo signal 32 due to the FID signal 31 overlaps with the original echo signal 28, and the image resulting from this echo signal 32 turns into the artifact. Accordingly, image quality of the resulting tomogram gets deteriorated.